A single modern semiconductor integrated circuit (IC), commonly referred to as a chip, frequently contains both one or more analog circuits and one or more digital circuits. Combining analog circuits and digital circuits within the same IC is commonly called a system-on-a-chip (SoC). An IC that implements a SoC provides significant advantages for the electronic device that includes such a chip. These advantages include: increased speed and reduced power, because more of the electronic device's signals stay within the SoC IC. These advantages also include reduced cost and reduced area requirements, because more of the electronic device is implemented within each SoC IC than would be implemented using a larger quantity of special purpose ICs.
However, implementing both an analog circuit and a digital circuit within the same IC poses very substantial design challenges. One of the foremost among these challenges is that electronic noise generated in the digital circuit generally transfers over to the analog circuit. Such electronic noise can easily interfere with the proper operation of the analog circuit. Such a noise transfer is also known as a coupling or an injection.
A digital circuit generally operates based on large swings in the voltages of its signal wires and clock wires. These large swings can produce substantial electrical noise in the power supply lines (i.e., Vdd lines), in the signal ground lines (i.e., Vss lines), and in the substrate of the IC (i.e., the package ground). A SoC IC may provide a digital Vdd and a digital Vss that are respectively kept separate from an analog Vdd and an analog Vss. However, a single IC must be implemented using a single substrate.
Digital circuits are generally more immune to electrical noise than are analog circuits. Thus, such noise is not usually enough to disrupt the operation of other digital circuits even if they are nearby. In contrast, analog circuits often operate with signals where relatively small voltage swings can be of crucial significance. Thus such noise may be enough to disrupt the proper functioning of an analog circuit even if a substantial distance separates the location of the analog circuit from the location of the digital circuit. In addition to noise transfer via Vdd, Vss, and the substrate, modern ICs generally have one or more seal rings that surround a central area of the IC. In conventional systems, seal rings are continuous metal rings that encircle the central area of the IC. The central area contains the entire circuitry of the IC including the analog and/or digital circuit, the ESD circuit, as well as pads for input/output (TO) signals, for power, and for ground.
The seal rings reduce the penetration of moisture and chemicals into the central area.
The seal rings also reduce mechanical stress on the central area. The IC manufacturing process generally includes a step in which a semiconductor wafer containing multiple copies of an IC is sawed into individual dies, where each die contains one copy of the IC. This reduction in mechanical stress is particularly important during the wafer sawing step, but is also beneficial during the steps between wafer sawing and completing the packaging of the IC.
Unfortunately, seal rings generally provide an electrical channel that transfers a significant amount of the electronic noise that is generated in the digital circuit to the analog circuit.
Thus, there is a need for a system that reduces transfer of electronic noise from a digital circuit to an analog circuit within the same IC, where the transfer is via the seal rings and the substrate. There is also a need for a method of manufacturing such a system.